1. Field of the Invention
The present invention relates to a motor driving technique and, more particularly, relates to a control when starting up a motor.
2. Description of the Related Art
In order to rotate a single-phase motor and a multi-phase motor at a desired torque, there is widely used a technique which controls an energization period of a motor coil by a pulse signal such as pulse width modulation signals.
In the case where driving of a stopped motor is started, when a switching voltage having a pulse width corresponding to a target torque is suddenly applied to a motor coil, an electric current is suddenly flown to the motor coil. The current flowing through the coil becomes a value in which the applied voltage is divided by a winding resistance because there is no generating function when starting up the motor, specifically, in the case where the number of rotation thereof is near to 0. A resistance value of the coil is designed to be very low in order to reduce electric power loss; therefore, the coil current exceeds the ratings of a driving circuit and the coil itself; and consequently, there is a case which provides influence on reliability of the circuit.
Furthermore, a counter electromotive voltage across the coil is proportional to a time changing rate of the current flowing through the coil. Therefore, as described above, when the coil current is rapidly increased, a very large counter electromotive voltage is generated, and the rating of the driving circuit is likely to be exceeded.
From such a reason, immediately after the start of starting up the motor, a soft start control which makes the current flowing through the coil increase gradually (for example, see Japanese Patent Application (Laid open) No. H7-95792, and Japanese Patent Application (Laid open) No. 2001-45790). In the soft start control disclosed in the Patent Document 1, a soft start voltage whose voltage value is slowly increased in response to time is generated, the soft start voltage is compared with a periodic voltage of a triangular wave or sawtooth wave pattern, a pulse modulation signal whose duty ratio is slowly increased is generated, an energization time of the coil is slowly increased, and the soft start is performed.
A voltage obtained by charging (discharging) a capacitor is used in generating the soft start voltage. When a power supply voltage is supplied to a fan motor driving circuit, the discharge of the capacitor is started, and a voltage slowly increasing or decreasing with time is generated.
In the case where the power supply voltage to be supplied to the fan motor driving circuit is shut down in a normal procedure, if a charge stored in the capacitor is discharged before the circuit is completely stopped, it is possible to generate the soft start voltage which is gradually increased from an initial voltage when the circuit is started up in the next.
However, when the power supply voltage to the fan motor driving circuit is instantaneously interrupted by an unexpected method different from the normal procedure, the circuit is shut down without discharging the charge stored in the capacitor. Since the charge stored in the capacitor loses its discharging path, the soft start voltage is continued to hold a certain value. In this case, when the fan motor driving circuit is started up in the next, the soft start voltage is started to increase from the value; and therefore, it is not possible to execute the desired soft start.